1. Field of The Invention
The subject invention relates to a circuit for deflecting an electron beam in a monitor.
2. Description of The Related Art
As shown in FIG. 1, a conventional deflection circuit includes a bipolar power transistor BJT, a damping diode D.sub.r, a retrace capacitor C.sub.r, a horizontal deflection coil L.sub.y, S-correction ID capacitor C.sub.s and a base driver circuit. The base driver circuit consists of a flyback isolation transformer T, damping resistors R1 and R2, a snubber circuit D1, R3 and C1, and a drive switch S1. FIG. 2 shows the key waveforms of the deflection circuit. In operation, the damping diode D.sub.r, which is in anti-parallel with the bipolar transistor BJT, is turned on at t.sub.0. The voltage V.sub.s is applied to the deflection coil L.sub.y and causes the deflection coil current i.sub.y to increase. The damping diode D.sub.r is turned off at zero voltage at t.sub.1, where the deflection coil current i.sub.y reverses. The transistor BJT is then turned on and takes the deflection coil current. The drive switch S1 is turned on at t.sub.2 and a negative voltage is applied to the base-emitter of the transistor BJT. After a storage time, the transistor BJT is turned off at t.sub.3. The deflection coil L.sub.y and the retrace capacitor C.sub.r form a resonant circuit which generates a high voltage for resetting the deflection coil L.sub.y. This high voltage V.sub.ce,max can be expressed as: ##EQU1##
where t.sub.s and t.sub.r are the sweep time and retrace time, respectively.
Because the power transistor BJT has a large tail current, this overlaps the collector-emitter voltage and generates a high switching turn-off loss. In addition, high voltage rating power transistors have a small current gain h.sub.fe (the ratio of the collector current to the base current), which requires a high based current to turn the device on and off. Thus, the power loss in the base drive circuit is significant.
The waveform of the base current for the transistor BJT has a great influence on its power loss. Unlike majority carrier power switches, such as MOSFETs and IGBTs, the BJT requires high base current to turn the device on and off, especially for high voltage and high current bipolar transistors, which have low current gains, usually less than 10. For monitor applications, the collector current of the BJT linearly increases with the sweep time, as shown in FIG. 2. Thus, the required base drive current should follow the collector current waveshape to achieve optimum operation of the BJT. In other words, it needs the highest base drive current at the end of the horizontal sweep at t2, where the BJT conducts the highest deflection coil current. Besides, since the horizontal deflection circuit operates at a wide rage of frequencies, typically from 30 kHz to 86 kHz, and even 100 kHz for high resolution, the base drive circuit should be able to supply proper base current for the operating frequency range. The conventional base drive circuit employs a flyback transformer to supply base current to the bipolar transistor BJT and uses high turns ratio to reduce the current level in the circuit on the primary side. As shown in FIG. 2, when S1 is turned on, a negative voltage, generated in the secondary winding, is applied to the emitter-base junction and turns off the transistor BJT. The supply voltage is supplied in the primary winding, which causes the magnetizing current to increase. When the drive switch S1 is turned off, the magnetizing current is reflected to the secondary winding and used as the base current to turn on the transistor BJT. Because of the nature of the flyback transformer, the base current linearly decreases with the sweep time. With the increase of the collector current, the base current provided by the drive circuit is actually decreased. This is contradictory to the requirements, this high base current being needed for the high collector current. If the base current is kept high enough at the end of the sweep time of t2 to fully conduct the deflection coil current, then the transistor BJT is definitely overdriven at the beginning of the transistor BJT conduction. As a result, high power dissipation exists and high rating components have to be used in the base drive circuit, which consequently increases the cost.